Photocatalytic composites containing titanium and limestone

ABSTRACT

New photocatalytic product comprising compounds of titanium integrated with limestone. The product is obtained by reacting limestone with a suitable precursor of titanium dioxide in a basic solution, followed by accurately washing the solid obtained, drying it and calcining it. A composite is obtained containing limestone, titanium dioxide and calcium titanate. The composite thus obtained, used as such or in mixture with other components, has shown an unexpectedly high photocatalytic activity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a §371 national stage entry of International Application No. PCT/EP2009/005571, filed Jul. 31, 2009, which claims priority to Italian Application No. MI2008A001445, filed Aug. 1, 2008, both of which are hereby incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention concerns the field of photocatalytic materials used for decontamination from environmental pollutants, and for preserving the original colour of articles of manufacture exposed to said pollutants, with application in particular in the field of cement.

STATE OF THE ART

The use of titanium dioxide in the form of anatase as photocatalyst in cement compositions is widely known. The resulting compositions are exploited to make various construction elements and articles of manufacture endowed with photocatalytic properties, capable of decomposing environmental pollutants in presence of light and oxygen. In these compositions titanium dioxide can be dispersed in bulk with the remaining components (WO-A-9805601, to the Applicant); alternatively, firstly a cement base free from titanium dioxide is formed, and then it is externally coated with titanium dioxide, optionally mixed with binders and/or adhesives of various types. In all these cases the titanium-containing photocatalyst is present in the form of a mere physical mixture with the mineral components of the cement composition. The interaction that is established in these cases is of the mechanical or weakly electrostatic kind, and thus there is no adequate continuity between photocatalyst and rest of the mixture. This can lead to various problems linked to inadequate interpenetration of the photocatalytic components and of those constituting the inert material. The close interaction between photocatalyst and mineral elements of the cement is however important for an effective photocatalytic action: indeed, in photocatalytic cements the cement component is known to simultaneously absorb atmospheric pollutants through a process of rapid dynamic equilibrium with the environment (adsorption/desorption): the pollutant temporarily adsorbed is then decomposed by the photocatalyst. However, in known products the adsorbent part and the photocatalytic part are clearly distinct: in this situation a part of the adsorbed pollutant can be desorbed before the photocatalyst is able to act sufficiently, with the consequence of an insufficient level of photocatalysis.

In an attempt to improve the degree of interaction between photocatalytic part and inert part, some materials have been proposed in which the titanium dioxide is supported on mineral components. An example of these products is titanium dioxide supported on metakaolin, described in patent application MI2007A002387, to the Applicant. However, as also highlighted in the aforementioned application in reference to various supports, the reactivity of the titanium dioxide varies greatly from depending on the support, and the properties of the resulting product are extremely variable and often unsatisfactory.

A high-performance photocatalytic is particularly desirable in the case of cement materials, characterised by a very low cost/weight ratio: for these materials, any increase in production costs linked to the addition of fine additives reflects greatly on said ratio, risking to make the end product unmarketable.

Currently the need for photocatalytic composites in which the photocatalytic part is greatly integrated with a mineral support material, which are endowed with a high photocatalytic activity, and possibly obtainable through a low-cost production process, thus remains widely unfulfilled.

Calcium titanate is a material with properties of refractoriness, chemical resistance and of a semi-conductor. It is found in nature in various forms (e.g. perovskite) characterised by a mixture of phases with different ratios between calcium and titanium, e.g. CaTiO₃, Ca₃Ti₂O₇, Ca₄Ti₃O₁₀, CaTi₄O₉, CaTi₂O₅, Ca₂TiO₄, CaTi₂O₄(OH)₂, etc. It can be prepared via dry- or wet route. Dry preparation is generally carried out by reacting titanium oxide and calcium carbonate at temperatures greater than 1300° C. (Izv. Akad. Nauk USSR-Neorg. Mater., 11 (1975) 1622). Wet preparation can be carried out in different ways, e.g. hydrothermically by heating an aqueous suspension of titanyl oxalate and a hydrated titanium gel to 150-200° C. in an autoclave (T. R. N. Kutty and R. Vivekanandam, Mater. Lett., 5 (1987) 79-83). It is also known to obtain calcium titanate via peroxide route by treating an aqueous calcium chloride and titanium chloride solution with hydrogen peroxide and ammonia, and subsequently calcining the precipitate obtained. (Pfaff, J. Eur. Ceram. Soc., 9, 1992, 293-299).

Mixtures of cement and titanates have occasionally been described. For example JP2000226248 describes cement mixtures with good flame and acid resistance containing a ceramic powder that includes potassium titanate and titanium dioxide.

SUMMARY

A new photocatalytic composite has now been identified in which the titanium is tightly and stably integrated with a mineral currently used in the field of cement, which is limestone. The composite is obtained by reacting a precursor of titanium dioxide with limestone in basic solution, followed by accurate washing, drying and calcining of the solid obtained. The composite contains limestone, titanium dioxide and calcium titanate, the latter characterised by two crystalline phases not known up to now (herein characterised and referred to as CT2 and CT5). The composite thus obtained, which can be used as such or in mixture with other components, has shown an unexpectedly high photocatalytic activity.

DESCRIPTION OF THE FIGURES

FIG. 1: Trend of the specific BET surface according to the treatment temperature of the photocatalytic composite STCA06.

FIG. 2: Diffractogram of the composite STCA 06

FIG. 3: Diffractogram of the acid residue of the composite STCA06

FIG. 4: Images in bright field TEM of a calcite crystal and of the micro-nanocrystalline aggregates

FIG. 5: High-resolution image of a grain of CT2 phase.

FIG. 6: Fourier transform of the image of FIG. 5 corresponding to the plane [100] with the main periodicities shown.

FIG. 7: High-resolution image of the grain rotated by 34.7° around the periodicity 0.99 nm (scale 2 nm)

FIG. 8: Fourier transform of the image of FIG. 7 corresponding to the plane [100] with the main periodicities shown.

FIG. 9: Images in bright field TEM of the crystals of phases CT2 (e1) and CT5 (a1, b1, c1, d1) (scale 50 nm).

FIG. 10: High-resolution image of a grain of CT5 phase obtained (scale 2 nm)

FIG. 11: Fourier transform of the image of FIG. 10 with the main periodicities shown.

FIG. 12: High-resolution image of a grain of CT5 phase (scale 2 nm)

FIG. 13: Fourier transform of the image of FIG. 12 with the main periodicities shown

FIG. 14: High-resolution image of a grain of CT5 phase (scale 2 nm).

FIG. 15: Fourier transform of the image of FIG. 14 with the main periodicities shown

FIG. 16: Abatement of NO over CEN mortar according to the type of photocatalyst. As such: CEN mortar with only Italbianco cement. As such CA-01: CEN mortar with Italbianco cement and limestone.

FIG. 17: Abatement of NO_(x) over CEN mortar for the composite STCA06 with respect to the cement.

FIG. 18: Abatement of NO over CEN mortar according to the treatment temperature.

FIG. 19: Abatement of the ethylbenzene by the tested products in proportion to the amount of titanium present.

DETAILED DESCRIPTION

The photocatalytic composite object of the invention comprises limestone, titanium dioxide and calcium titanate; the latter is present in part in the known form of perovskite (traces) and in part in the form of two new crystalline phases, herein identified and characterised for the first time, referred to as CT2 and CT5.

For the purposes of the present invention by calcium titanate in crystalline phase CT2 it is meant a crystalline chemical compound containing calcium and titanium, present in molar ratio 1:2, having empirical formula CaTi₂O₅, identified by the characteristic diffraction peaks: (002) d(interplanar distance)=4.959; (210-202) d=2.890; (013) d=2.762 and (310-122) d=2.138. These peaks are indexed with an orthorhombic cell having the following reticular parameters: a=7.1 Å, b=5.0 Å, c=9.9 Å.

For the purposes of the present invention by calcium titanate in crystalline phase CT5 it is meant a crystalline chemical compound containing calcium and titanium, present in molar ratio 1:5, having empirical formula CaTi5O11, identified by the characteristic diffraction peaks: (002) d=8.845; (023) d=4.217; (110) d=3.611 and (006) d=2.948. These peaks are indexed with an orthorhombic cell having the following reticular parameters: a=3.8 Å, b=12.1 Å, c=17.7 Å.

For the purposes of the present application the crystallographic parameters for phases CT2 and CT5 herein indicated and claimed are meant to be variable within a range of about ±0.5 Å for the parameters of the cell a, b, c, and within a range of about ±0.05 for the interplanar distances d; similarly, the calcium:titanium molar ratios indicated above are meant to be variable by about ±10%.

The microstructural characteristics of phases CT2 and CT5 are widely illustrated in the experimental part.

In the composites object of the invention, the amounts of calcium titanate in CT2 phase and in CT5 phase are widely variable: in general, the CT2 phase is present in a greater amount than CT5; for example the weight ratio CT2:CT5 is at least 60:40, or at least 80:20, or at least 95:5. In an embodiment of the invention the calcium titanate is present exclusively in CT2 phase or exclusively in CT5 phase.

The aforementioned calcium titanate in CT2 and/or CT5 phase represents per se a particular embodiment of the present invention.

In the present composite, the calcium titanate is accompanied by other components, in particular limestone and titanium dioxide, the latter in mixed anatase and rutile form. The limestone used to form the composite is the one commercially available, preferably in finely divided form, also commercially available (example source: cava di Tinella (Fasano, Brindisi))

The BET surface area of the composite generally ranges from 10 to 150 m²/g, with preferred values between 15 and 50 m²/g, e.g. between 20 and 30 m²/g.

The process for obtaining the composites described here constitutes a further aspect of the invention. It generally comprises reacting limestone and a precursor of titanium dioxide in a basic aqueous solution. The reactants can be added into the reactor in indifferent order; preferably the limestone is contacted first with the basic solution and then with the precursor. The precursor used is preferably titanyl sulphate. In the process conditions the precursor converts in part into titanium dioxide and in part into calcium titanate described above. Preferably, an amount of precursor is used corresponding to a theoretical content (i.e. calculated considering a total conversion of the precursor into TiO₂) of about 40% by weight with respect to the limestone. The basic solution is made such through use, for example, of NaOH. The reaction goes on for a time of between 45 and 90 minutes, at a temperature ranging from 20 and 80° C. At the end of the reaction, the resulting solid product is recovered from the solution, carefully washed with water until neutrality, dried and calcined. If sodium-containing reactants are used (e.g. NaOH), the sodium residue in the solid product subjected to drying must be less than 0.05% by weight (expressed as Na₂O over the dry product); such a condition is easily obtained for example by washing the recovered product until neutrality.

The calcining preferably takes place at a temperature ranging from 300 to 800° C., e.g. between 450 and 700° C.; particularly effective photocatalytic composites have been obtained by calcining at about 650° C. The choice of the best calcining temperatures highlights a particularly unexpected aspect of the present invention. Indeed, it is per se known (and even experimentally confirmed, see FIG. 1) that the increase in calcining temperature results in a reduction of the specific surface of the calcined product; on the other hand, photocatalysis is a typical surface phenomenon and such phenomena are predictably disadvantaged in conditions of low contact surface; surprisingly however, the present invention has shown the opposite tendency, by identifying the most photocatalytically active composites in the lower ranges of specific surface, obtainable by calcining in the upper temperature ranges.

A further object of the present invention is the photocatalytic composite obtained through the process described above.

From the point of view of the elemental composition (as detectable by X-ray fluorescence and atomic absorption), the composites according to the invention can be further characterised as follows:

Calcium (expressed as CaO) 20-50% Titanium (expressed as TiO₂) 15-68% Sulphur (expressed as SO₃)  2-12% Sodium (expressed as Na₂O) <0.05% L.o.I. (*)  3-40% (*): loss on ignition

Or more preferably as follows:

Calcium (expressed as CaO) 26.9% Titanium (expressed as TiO₂) 51.7% Sulphur (expressed as SO₃) 7.54% Sodium (expressed as Na₂O) <0.01%  L.o.I. (*) 13.4%

The elemental composition given in the tables refers to the composite as a whole: such a composite comprises, in addition to calcium titanate, limestone, titanium dioxide and possible residues of the reactants used for the for titanate-forming reaction.

However, as can be seen from the tables, a characteristic of the composite is that it is substantially free from sodium residues (i.e. having a sodium percentage, expressed as Na₂O, below 0.05% by weight over the dry product). Such a characteristic, obtainable by carrying out prolonged and repeated washes of the reaction precipitate, is responsible for the formation of significant amounts of titanium dioxide in the composite. Otherwise, composites obtained similarly, but without elimination of the sodium residues, were substantially free from titanium dioxide: the latter family of composites has specific application advantages and is the object of a co-pending application to the Applicant.

As highlighted by the electron microscopy observations contained in the experimental part, the titanium dioxide and the calcium titanate are in the form of crystalline grains of a size of about 10-150 microns, closely connected to limestone grains. There is thus clearly a strong aggregative link between the photocatalytic portion of the composite (titanium) and the mineral support component (limestone); within these aggregates, the calcium titanate crystals in phase CT2 are generally rounded, whereas those in phase CT5 generally have a characteristic rod shape.

The present invention represents a successful example of composite material in which different titanium-containing compounds are closely and stably linked to a support material (limestone) able to be used in the cement field. The close interconnection between the photocatalytic and non-photocatalytic parts of the composite obtains a substantial continuity between absorption sites of the pollutants and decomposition sites thereof, with the advantage of high photocatalytic efficiency. Such efficiency has been highlighted by abatement tests of N-oxides (NO_(x)) and VOC (aromatic hydrocarbons), using the composite of the invention either as such, or incorporated in bulk in a cement matrix.

A further object of the invention is therefore the use of the photocatalytic composite described earlier as photocatalytic product as such, or in the preparation of cements and cement articles of manufacture endowed with photocatalytic activity. The article of manufacture can contain the composite dispersed in bulk, or layered on its outer surfaces, as a coating: in the latter case the photocatalytic composite is preferably mixed with suitable tackifiers, used to promote suitable cohesion between article of manufacture and coating layer. In any case, the composite is used in amounts such as to obtain a concentration of composite in bulk preferably ranging from 1% to 10%, more preferably between 2.5% and 8.5%. The methods for the dispersion in bulk or for the outer coating are per se widely known in the field in question.

An aspect of the invention thus concerns photocatalytic composition, in particular of cementitious type, comprising the composite described above. The further elements of the cement composition are those commonly known, in particular: various hydraulic binders, optional aggregates and additives used in the cement field. The hydraulic binders and the aggregates (defined for example by standards UNI ENV 197.1 and UNI 8520) are products widely known in the field. The compositions according to the invention can be provided in fluid state, or else mixed with water (to form mortars or concretes, depending upon the granule size of the aggregates used), or else they can be provided in the corresponding forms free from water (dry premixes). These compositions are used to form photocatalytic articles of manufacture through suitable casting in moulds and similar technologies; the resulting articles of manufacture contain the composite of the invention dispersed in bulk. Alternatively, they can be used as coating formulations of pre-existing articles of manufacture, preferably co-formulated with suitable tackifiers.

The invention is illustrated hereafter not for limiting purposes through the following examples.

EXPERIMENTAL PART Example 1 Preparation of the Composite (STCA06)

45 g of a commercial calcareous filler (origin: cava Tinella di Brindisi) were stirred, suspended in 160 ml of a NaOH solution (200 g/l in distilled water), and an aqueous solution of 300 ml of TiOSO₄ (100 g/l of TiO₂), so as to have a theoretical TiO₂ content equal to about 40% by weight, was dripped. After centrifuging and washings with distilled water the powder was dried at 105° C. in a ventilated oven. Before performing the calcining heat treatment at 450° C. for 2 hours, the product was broken up so as to obtain a powder. Further samples of the same filler were treated in the same way, calcining at 550 and 650° C.

Example 2 Microstructural Characterisation

The composite STCA 06 obtained in example 1 (calcining temperature 650° C.), subjected to diffractometric analysis (diffractometer BRUKER D8 Advance and CuKα (λ_(Cu)=1.545 Å radiation) proved to be a polyphase mixture composed of calcite, traces of perovskite, titanium dioxide, and calcium titanate in different crystalline phases. In particular, the diffraction profile showed the presence of a series of peaks not attributable to known crystalline phases, which can be referred to two different phases that proved to be calcium titanate-containing “compounds” with Ca:Ti ratios of 1:2 and 1:5 respectively (FIG. 2).

The accurate position of the peaks of the new crystalline phases was determined through diffractometric analysis of the sample after elimination of the calcite by treatment in diluted HCl (1:10) and subsequent drying at 60° C. (FIG. 3).

The observed interplanar distances (d) of the two phases are shown in the following tables, wherein h, k, l indicate the Miller indices, and °2θ indicates the diffraction angle.

interplanar distances for CaTi₂O₅; Space group: Pna2₁ a = 7.1 Å, b = 5.0 Å, c = 9.9 Å h k l d °2θ 0 0 2 4.96 17.87 0 1 1 4.48 19.80 1 1 0 4.10 21.66 1 1 1 3.79 23.46 2 0  0* 3.55 25.08 2 0 1 3.34 26.67 1 1 2 3.16 28.22 2 1 0 2.90 30.84 2 0 2 2.89 30.97 2 1 1 2.78 32.16 0 1 3 2.76 32.39 1 1 3 2.57 34.83 0 2 0 2.51 35.72 2 1 2 2.50 35.86 0 0 4 2.48 36.19 2 0 3 2.42 37.14 1 2 0 2.37 37.98 1 2 1 2.30 39.09 0 2 2 2.24 40.22 2 1 3 2.18 41.40 3 1 0 2.14 42.20 1 2 2 2.14 42.27 1 1 4 2.12 42.57 3 1 1 2.09 43.22 2 2 0 2.05 44.15 2 0 4 2.03 44.54 2 2 1 2.01 45.13 3 1 2 1.96 46.17 1 2 3 1.92 47.18 2 2  2* 1.89 47.99 2 1 4 1.88 48.27 0 1 5 1.85 49.35 3 1 3 1.80 50.79 1 1 5 1.79 51.11 4 0 0 1.77 51.48 0 2 4 1.76 51.77 4 0 1 1.75 52.36 2 2 3 1.74 52.48 2 0 5 1.73 52.83 3 2 0 1.72 53.16 1 2 4 1.71 53.47 3 2 1 1.70 54.02 4 1 0 1.67 54.85 4 0 2 1.67 54.94 0 0 6 1.65 55.54 0 3 1 1.65 55.63 4 1 1 1.65 55.69 2 1 5 1.64 56.14 1 3 0 1.63 56.42 3 2 2 1.63 56.54 3 1 4 1.62 56.78 1 3 1 1.61 57.25 4 1 2 1.58 58.17 2 2 4 1.58 58.36 4 0 3 1.56 59.06 1 3 2 1.55 59.68 1 1 6 1.53 60.32 3 2 3 1.53 60.59 1 2 5 1.52 60.87 2 3 0 1.51 61.16 2 0 6 1.50 61.87 2 3 1 1.50 61.95 0 3 3 1.49 62.09 4 1 3 1.49 62.15 1 3 3 1.46 63.61 3 1 5 1.45 63.94 4 2 0 1.45 64.24 2 3 2 1.45 64.27 4 0 4 1.44 64.55 2 1 6 1.44 64.88 4 2 1 1.43 65.01 2 2 5 1.43 65.42 3 2 4 1.41 66.00 4 2 2 1.39 67.27 4 1 4 1.39 67.50 0 2 6 1.38 67.81 2 3 3 1.38 68.05 3 3 0 1.37 68.63 5 1 0 1.37 68.69 0 1 7 1.36 68.78 1 3 4 1.36 68.89 1 2 6 1.36 69.26 3 3 1 1.35 69.37 5 1 1 1.35 69.43 1 1 7 1.34 70.22 4 2 3 1.33 70.97 4 0 5 1.32 71.26 3 3 2 1.32 71.57 5 1 2 1.32 71.62 2 0 7 1.32 71.65 *peaks on top of the main peaks of the anatase.

interplanar distances for CaTi₅O₁₁; Space group: Cmcm a = 3.8 Å, b = 12.1 Å, c = 17.7 Å h k l d °2θ 0 0 2 8.85 9.99 0 2 0 6.04 14.66 0 2 1 5.71 15.50 0 2 2 4.99 17.77 0 0 4 4.43 20.05 0 2 3 4.22 21.04 1 1 0 3.61 24.62 0 2 4 3.57 24.93 1 1 1 3.54 25.13 1 1 2 3.35 26.63 1 1 3 3.08 28.95 0 2 5 3.05 29.22 0 4 0 3.02 29.58 0 4 1 2.97 30.01 0 0 6 2.95 30.27 0 4 2 2.86 31.29 1 1 4 2.80 31.95 1 3 0 2.76 32.44 1 3 1 2.72 32.84 0 4 3 2.69 33.32 0 2 6 2.65 33.79 1 3 2 2.63 34.02 1 1 5 2.53 35.47 1 3 3 2.50 35.92 0 4 4 2.49 35.99 1 3 4 2.34 38.43 0 2 7 2.33 38.57 0 4 5 2.30 39.19 1 1 6 2.29 39.40 0 0 8 2.21 40.75 1 3 5 2.18 41.47 0 4 6 2.11 42.83 0 2 8 2.08 43.53 1 1 7 2.07 43.65 1 5 0 2.04 44.47 1 5 1 2.02 44.78 1 3 6 2.01 44.96 0 6 0 2.01 45.02 0 6 1 2.00 45.33 1 5 2 1.98 45.69 0 6 2 1.96 46.24 0 4 7 1.94 46.83 1 5 3 1.92 47.19 0 6 3 1.90 47.72 2 0 0 1.89 48.01 1 1 8 1.89 48.19 0 2 9 1.87 48.65 1 3 7 1.86 48.82 2 0 2 1.85 49.17 1 5 4 1.85 49.23 0 6 4 1.83 49.74 2 2 0 1.81 50.48 2 2 1 1.80 50.76 0 4 8 1.78 51.15 2 2 2 1.77 51.59 0 0 10 1.77 51.59 1 5 5 1.76 51.76 0 6 5 1.75 52.26 2 0 4 1.74 52.53 2 2 3 1.73 52.96 1 1 9 1.73 52.96 1 3 8 1.73 53.02 0 2 10 1.70 53.94 1 5 6 1.68 54.74 2 2 4 1.67 54.84 0 6 6 1.66 55.22 0 4 9 1.65 55.74 2 2 5 1.61 57.20 2 4 0 1.60 57.41 1 3 9 1.60 57.51 2 4 1 1.60 57.66 2 0 6 1.59 57.82 1 1 10 1.59 57.97 1 5 7 1.59 58.13 2 4 2 1.58 58.43 0 6 7 1.57 58.59 1 7 0 1.57 58.79 1 7 1 1.56 59.04 0 2 11 1.55 59.39 2 4 3 1.55 59.70 1 7 2 1.55 59.80 2 2 6 1.54 60.00 0 4 10 1.53 60.60 1 7 3 1.52 61.05 0 8 0 1.51 61.39 2 4 4 1.51 61.44 0 8 1 1.50 61.64 1 5 8 1.50 61.88 1 3 10 1.49 62.28 0 6 8 1.49 62.33 0 8 2 1.49 62.38 1 7 4 1.48 62.77 0 0 12 1.48 62.96 2 2 7 1.47 63.20 1 1 11 1.47 63.20 0 8 3 1.46 63.60 2 4 5 1.46 63.64 2 0 8 1.44 64.75 1 7 5 1.43 64.94 0 2 12 1.43 65.04 0 8 4 1.43 65.28 0 4 11 1.42 65.71 1 5 9 1.41 65.99 2 4 6 1.41 66.28 0 6 9 1.41 66.42 2 2 8 1.40 66.80 1 3 11 1.39 67.31 0 8 5 1.39 67.41 1 7 6 1.39 67.55 2 6 0 1.38 67.93 2 6 1 1.37 68.16 1 1 12 1.37 68.67 2 6 2 1.36 68.86 2 4 7 1.35 69.32 0 8 6 1.34 69.98 2 6 3 1.34 70.02 1 5 10 1.34 70.43 1 7 7 1.33 70.57 2 2 9 1.33 70.75 0 6 10 1.33 70.85

Example 3 Microscope Analysis

In order to better understand the nature of the sample, both the sample as such and the acid residue, were subjected to analysis by transmission electron microscopy (TEM). The observations allowed to establish that the sample is composed of a mixture of crystals of a few microns of calcium carbonate, surrounded by micro-nano crystalline aggregates (grains) of calcium titanate and carbonate having variable size from 10 nm to 150 nm (see FIG. 4).

Through microanalysis with EDS detector it has been possible to identify, as majority constituents, two families of crystals, one containing exclusively Ti, which proved to be anatase crystals, and one with a slightly rounded shape containing both Ca and Ti. The semiquantitative analyses made by focussing the electron beam on different crystals of the latter phase have allowed to establish that the Ca:Ti ratio in this phase, here referred to as CT2, is about 1:2

High-resolution images were carried out on some crystals of this phase with the corresponding Fourier transforms (FIGS. 5-8), from which it has been possible to extract information on the cell parameters for the CT2 phase:

Orthorhombic: a=7.1 Å, b=5.0 Å, c=9.9 Å.

The extinction conditions observed are the following: 0k1 k+1=2n hh1 no cond  (1) 2hh1 no cond h00 h=2n 0k0 k=2n  (2)

By adding the extinctions (1) and (2) one obtains as possible space group Pna21 (Herman Mauguin Symbol), corresponding to the space group 33 shown in International Tables of Crystallography, vol. A, “Space Groups Symmetry”, V ed., Kluver Acad. Publ. 2002)

Possible monoclinic distortions can exist and the TEM data obtained cannot exclude them.

The software used for simultaneously indexing such patterns was QED (Belletti D., Calestani G., Gemmi M, Migliori A.—QED V1.0: a software package for quantitative electron diffraction data treatment—Ultramicroscopy, 81 (2000) pp 57-65).

In light of the information obtained on the cell of this new phase it was possible to assign some of the peaks not identified in the diffractogram of the sample STCA06 to the CT2 phase.

The remaining peaks are attributable to a different phase (CT5, see later).

The cell parameters of the CT2 phase have been further refined through fitting of the calculated diffractometric profile with the real one.

Through microanalyses with EDS detector it has been confirmed that the family of rounded crystals is conform with CT2 phase, found in the sample of photocatalytic composite.

Other crystals of characteristic elongated form (FIG. 9) were found to contain Ca, Ti and small amounts of Na. This new crystalline phase, characterised by a Ca:Ti ratio of about 1:5, is referred to here as CT5. Similarly to what was done for CT2, some high-resolution images were taken, with the corresponding Fourier transforms (see FIG. 10-15) from which it has been possible to extract information on the cell parameters.

The main characteristic of this phase is a periodicity of 17.6 Å.

From the simultaneous indexing of such patterns via QED software (Belletti et al., op. cit.) it has been possible to derive a possible cell for the compound in question. The cell was orthorhombic C centred:

a=3.75 (10) Å, b=11.85 (20) Å, c=17.6 (2) Å (decimal error)

Possible monoclinic distortions can exist and the TEM data obtained cannot exclude them.

The extinction conditions observed are: hk1 h+k=2n hk0 h+k=2n 0k1 not able to be determined h01 h,1=2n h00 h=2n 0k0 k=2n 001 1=2n

These extinctions are compatible with the following possible space groups:

type C-c-: Cmc21, C2 cm, Cmcm (corresponding to the space group 63, cf. International Tables of Crystallography, vol. A, “Space Groups Symmetry”, V ed., Kluver Acad. Publ. 2002) in the case of extinction 0k1 k=2n;

type Ccc-:Ccc2, Cccm in the case of extinction 0k1 k,1=2n.

The cell parameters of the CT5 phase have been further refined through fitting of the calculated diffractometric profile with the real one.

Example 4 Analysis of Specific Bet Surface and Microporosity

The values measured during the analysis of the new photocatalytic composite STCA 06 shown in the table show an increase in the specific surface of the heat treated product, with respect to the limestone as such, with a substantial increase in the non-microporous fraction

S.S.A Micropore S.S.A non- BET Volume Micropores micropores m²/g mm²/g m²/g m²/g STCA 06 24.69 0 0 24.69 CA - 0.84 0.01 0.03 0.81 limestone

The analysis of the influence of the heat treatment on the specific surface (FIG. 1), carried out on the sample of photocatalytic composite, shows a linear decrease with the temperature as shown in the table according to the function:

BET = −0.4312 × T + 302.61 (m²/g) 450 < T < 650 Calcining T ° 450 550 650 BET m²/g 110.93 60.72 24.69

Example 5 Photocatalytic Activity on Cement: NO_(x) Abatement Measurements

The composite STCA06 was mixed with white cement (Italbianco 52.5 di Rezzato) so as to obtain photocatalytic cements with percentage by weight of photocatalyst within the range 2.5-8.5%. NO_(x) abatement analyses were carried out on cement mortars made with normalized sand CEN (according to UNI 196-1) by preparing tests in Petri dishes of diameter 8 cm and surface of about 60 cm². The results obtained show an excellent behaviour of such cements, comparable to that of cement containing commercial anatase (FIG. 16).

The abatement values measured on the mortars CEN containing the composite STCA06 at different concentrations on cement have shown good NO_(x) abatement values already at percentages of around 2.5% by weight. (See FIG. 17)

The treatment temperature proved important for photocatalytic activity. Indeed, it was observed that there is a progressive increase in activity as the temperature increases, as highlighted by observations at 450, 550 and 650° C. (FIG. 18).

Example 6 Photocatalytic Activity on Cement: VOC Abatement Measurements

The evaluation of the abatement capability of aromatic hydrocarbons was carried out using the pure photocatalytic products (not mixed with cement) under UV radiation. Ethylbenzene was used as organic substance, using a flow apparatus typical of tests on catalysts (oxidation of ethylbenzene in air). In this way the intrinsic activity of the material is determined, disregarding the diffusive phenomena. The results obtained show an excellent level of abatement activity of the product. It is even greater than the best commercial TiO₂ (FIG. 19). 

The invention claimed is:
 1. Photocatalytic composite comprising limestone, titanium dioxide, and calcium titanate in the crystalline phases CT2 and/or CT5 characterised by the following diffraction peaks: CT2: (002) d=4.959; (210-202) d=2.890; (013) d=2.762 and (310-122) d=2.138; CT5: (002) d=8.845; (023) d=4.217; (110) d=3.611 and (006) d=2.948, wherein the calcium titanate in CT2 phase has empirical formula CaTi₂O₅, and the calcium titanate in CT5 phase has empirical formula CaTi₅O₁₁.
 2. Composite according to claim 1, wherein said peaks of the CT2 phase are indexed with an orthorhombic cell having the following reticular parameters: a=7.1 Å, b=5.0 Å, c=9.9 Å.
 3. Composite according to claim 1, wherein said peaks of the CT5 phase are indexed with an orthorhombic cell having the following reticular parameters: a=3.8 Å, b=12.1 Å, c=17.7 Å.
 4. Composite according to claim 1, wherein the CT2 phase is present in an amount higher than the CT5 phase.
 5. Composite according to claim 1, having specific BET surface ranging from 10 to 150 m²/g.
 6. Composite according to claim 5, having specific BET surface ranging from 15 to 50 m²/g.
 7. Composite according to claim 6, having specific BET surface ranging from 20 to 30 m²/g.
 8. Calcium titanate with photocatalytic activity, characterised by the presence of the crystalline phases CT2 and/or CT5, as described in claim
 1. 9. Process to obtain the composite described in claim 1, comprising reacting limestone and a precursor of titanium dioxide in presence of a basic aqueous solution, recovering the solid product thus obtained, washing it until neutrality, drying it and calcining it.
 10. Process according to claim 9, wherein the precursor is titanyl sulphate, the basic solution contains NaOH, and the solid product is calcined at a temperature ranging from 300 to 800° C.
 11. Process according to claim 10, wherein the solid product is calcined at a temperature ranging from 450 to 700° C.
 12. Cement composition comprising the photocatalytic composite described in claim 1, water, a hydraulic binder, and optionally aggregates.
 13. Dry premix comprising the photocatalytic composite described in claim 1, a hydraulic binder, and optionally aggregates.
 14. Photocatalytic article of manufacture comprising, dispersed in bulk or layered on its surface, the composite described in claim
 1. 